Among a variety of blood pressure gauges having been proposed, one is configured to increase the pressure within the cuff to a predetermined level and then gradually decrease the pressure, during which the blood pressure of the subject is measured. A flow control valve for use in such a blood pressure gauge to gradually decrease the pressure within the cuff is disclosed, e.g., in Japanese Patent Laying-Open No. 6-47008 titled “Flow Control Valve”. The flow control valve described therein includes a front case with a pressure inflow port (gas inflow port) and a pressure outflow port (gas outflow port) formed therein. A driving shaft is supported such that it can move toward and away from the gas inflow port, and an orifice packing is attached to a portion of the driving shaft opposite to the gas inflow port. A magnet coil is further attached to the driving shaft, and a plate and a yoke excited by a permanent magnet are arranged around the magnet coil. The front and back portions of the driving shaft are connected to a frame portion via front and back dampers, respectively.
With this flow control valve, when current is passed through the magnet coil, the driving shaft moves together with the magnet coil by an electromagnetic force generated by the permanent magnet and the magnet coil, and the orifice packing closes the gas inflow port. This kind of control valve is called a “moving coil type”, since the magnet coil moves.
The flow control valve of this type, however, is disadvantage in that it has many complex parts and requires manpower for assembly, hindering the use of an auto-assembly machine. This increases the parts cost and degrades the productivity.
To solve such problems, the applicant conceived a flow control valve as shown in FIG. 15A (schematic cross sectional view) and FIG. 15B (left side view), and filed an earlier application (Japanese Patent Application No. 2000-31920). The flow control valve disclosed therein is of a moving magnet type with a permanent magnet made to move.
In the flow control valve shown in FIGS. 15A and 15B, a housing is formed of a front cap 2, a front case 8 and a frame lid 10. The housing has a gas inflow port 1a with an inner tube 1 of a nozzle form opening within, and a gas outflow port 1b communicating with gas inflow port 1a via an internal space. An actuating shaft 4 is arranged within the housing such that it can move toward and away from inflow port 1a. An orifice packing 3 is attached to an end 4a of actuating shaft 4 opposite to inflow port 1a so that the movement of actuating shaft 4 opens and closes inflow port 1a. A permanent magnet 5 is provided to press actuating shaft 4. Permanent magnet 5 is movably passed through a hollow portion of a bobbin 7 provided with a magnet coil 6. Magnet coil 6 is connected to an external terminal 11. Actuating shaft 4 is configured to move leftward in FIG. 15A by an electromagnetic force generated by permanent magnet 5 and magnet coil 6.
Further, actuating shaft 4 is connected to front cap 2 via a damper 9. Damper 9 biases the shaft 4 to the right direction of FIG. 15A. The end surface of orifice packing 3 and the opening surface of inflow port 1a are both flat. The end surface of orifice packing 3 is made diagonal (non-parallel) to the opening surface of inflow port 1a, at an angle of, e.g., 3°. The hollow portion of bobbin 7 has a wall serving as a stopper of permanent magnet 5, and an air vent 7b is formed to allow smooth movement of permanent magnet 5.
A case where the flow control valve configured as described above is utilized for measurement of blood pressure is now described by way of example. The schematic configuration of the blood pressure gauge is shown in FIG. 13, and the relevant flow control valve is used as a gradual exhaust valve 36 in FIG. 13. Firstly, a current of a predetermined level is passed through magnet coil 6 to generate an electromagnetic force by the interaction with permanent magnet 5. This electromagnetic force causes permanent magnet 5 to move to the left, thereby pressing actuating shaft 4. Correspondingly, orifice packing 3 at the end 4a of the actuating shaft is pressed to contact inflow port 1a, so that inner tube 1 attains a completely blocked state.
Next, under the blocked state, a pump is activated to introduce air into a cuff for pressurization. This is followed by a cuff depressurizing process, during which a current supplied to magnet coil 6 is gradually decreased to progressively reduce a thrust by the electromagnetic force. Accordingly, orifice packing 3 moves to the right direction by means of the spring action of damper 9 and the slant repulsive action of orifice packing 3. This gradually opens inflow port 1a, and the air within the cuff is exhausted very slowly into the atmosphere from inflow port 1a via outflow port 1b. The blood pressure of the subject is measured during this process.
As described above, with the flow control valve shown in FIGS. 15A and 15B, the thrust by the electromagnetic force generated at magnet coil 6 is used to shut inflow port 1a, and the repulsive forces of damper 9 and of orifice packing 3 itself are used to open inflow port 1a. The thrust by the electromagnetic force and the repulsion forces are transmitted to orifice packing 3 via permanent magnet 5 and actuating shaft 4. The structure shown in FIGS. 15A and 15B, however, is disadvantageous in that it cannot effectively utilize the thrust by the electromagnetic force generated by magnet coil 6. The flow control valve requiring large valve load (pressing force of orifice packing 3 against inflow port 1a) would increase in size and require huge electric power.
In other words, with the flow control valve shown in FIGS. 15A and 15B, the force to press orifice packing 3 against inflow port 1a is not so strong. Such a weak pressing force is insufficient for a blood pressure gauge applied to the arm which receives greater pressure than a blood pressure gauge applied to the wrist, possibly resulting in insufficient blocking of inflow port 1a when pressurizing the cuff to a level greater than maximum blood pressure. If it is attempted to increase the pressing force to avoid this unfavorable situation, the flow control valve would become large, and the consumed power would increase accordingly.
Further, with the flow control valve shown in FIGS. 15A and 15B, brittle permanent magnet 5 would often get chipped due to shock of drop or the like, resulting in poor controllability of the flow rate.